I have a 1999 Lincoln Navigator that just started having an airflow problem with the A/C. While the A/C is on and the air is coming from the panel, it will start switching the airflow back and forth between the dashboard defrost and the panel. It switches for about 10 seconds, then comes back to the panel.

I've never had this problem before in any of my cars and wanted to see if anyone else may know what's going on here?? Any advise/recommendations is very much appreciated! Thanks